Heat exchange methods



May 9, 1961 c. DREYER ET AL HEAT EXCHANGE METHODS 4 Sheets-Sheet 1 Filed March 1, 1956 E E z E I B F F F F F F F Q m Q 0 O 0 0 0 0 0 0 D 0 O 0 O m 0 O 0 0 6 0 1 EIO lIO lIO IIO IIO 2 A [6 6 l l w. 2 3 4 5 6 6 6 6 o I I I I I I I OI m Q 0 G A m m 1 d k R E 0 6 UNEHM m 6 G mw M 1 F 6 N W M EW E m I R l v ww F o NM wW o 5 m Rm o M 0 w wP uc w o o wamwf l0 1 2 3 CRBTMFG 8 6 2 O 8 6 2 O 8 6 3 3 M 3 3 2 2 M 2 2 1 1 SYSTEM PRESSURE INVENTORS cum/s DREYER Fl 6.1 EUGENE E. HARRISON M/LTON wow/a ATTORNIEYSV May 9, 1961 c. DREYER ET AL HEAT EXCHANGE METHODS 4 Sheets-Sheet 2 Filed March 1, 1956 WORKING PRESSURE CURVE SHOWING RELATIONSHIP BETWEEN PRESSURE, TEMPERATURE, AND MASS FOR A WATER FILLED SYSTEM HAVING 600 CU. FT. VOLU M (NOTE; THIS IS AN EXPANSION OF FIG.1.)

SYSTEM PRESSURE INVENTORS CURTIS DRE YER FIG 2 EUGENE E. HARR/SON MILTON LUDW/G 4% ATTORNEYS May 9, 196] C. DREYER ET AL HEAT EXCHANGE METHODS Filed March 1, 1956 4 Sheets-Sheet 5 /4 u D f i 5 HEAT ADDITION EITEMPERATURE s ZONE 1 m HEAT WITHDRAWAL ZONE 4 FUEL I) FLOW RATE o I I 0 I I- 1 20 E )fi-H 5 CIRCULATING IN PUMP 22 3:1

WATER OUT INVENTORS CURTIS DREYER sues/v5 E. HARR/SON M/L ro/v wow/c ATTORNEYS y 9, 1961 c. DREYER ET AL 2,983,449

HEAT EXCHANGE METHODS Filed March 1, 1956 4 Sheets-Sheet 4 I 12 I I REACTANTS VENT INVENTORS CURTIS DREVER 4 EUGENE E. HARR/SON MILTON LUDW/G ATTQRNEYS 2,983,449 HEAT EXCHANGEMETHODS Curtis Dreyer, Oakland, Eugene E. Harrison, Berkeley, and Milton Ludwig, Kensington, Calif., assignors, by

mesne assignments, to California Research Corporation,

San Francisco, Calif., a corporation of Delaware Filed Mar. 1, 1956, Ser. No. 568,829 h 3 Claims. orgy-'1 This invention relates to methods of operating a heat exchange system utilizing a* circulatin g stream of liquid in a closed loop which includes both a heat-addition zone and one or more heat-withdrawal zones, and particularly refers to methods of stabilizing and controlling the opera tion of such systems which utilize specific properties of the liquid medium employed at pressures and temperatures approaching its critical point.

Certain endothermic-chemical reactions involving organic materials which are carried out in confined zones or reactors at pressures varying up to about 300.0 p.s.i,a. (pounds per square inch, absolute) and at temperatures rangingup to about 700 F., require very accurate control of both the temperature and pressure of the reaction zone to obtainsafe and economical stable operation. An: example is that of US. Patent 2,722,549, entitled, Oxidae,

tion Process, for producing phthalic acids from hydrocarbons Physical limitations of structural materials used to confine such reaction zones include tensile strength:

properties, resistance to corrosion at the conditions en: countered, reactivityof reacting materials with heating fluids in case of equipment leaks or failure, and other fac-' tors known in the chemical processing art. To add the heat required for the endothermic reaction, to preheat the reacting materials, to compensate for heat losses, etc, it is contemplated to provide a confined stream of circulat-f ing fluid in a closed loop, one part of which may be a tubular furnace for adding heat, and another part of which.

may be one or more tubular heat exchangers or reactors: of suitable construction. Circulation is established by ap-.

propriate pumps, usually of the centrifugal type.

Heretofore, liquid media for heat transfer under tem perature conditions above about 450 F. have been petro leum oil fractions at atmospheric pressuretor ranges from 300 F. to about 550-600 F., above which coking occurs. Other liquids, such as Dowtherm, ordinarily used as the eutectic mixture of 73.5% diphenyl oxide and 26.5% diphenyl, are usable as a liquid up to 500'F.-

at atmospheric pressure and as a vapor up to 100 p.s.i.g.f. and 700 F. Molten salts and molten metals, e.g., NaK

eutectic, have been used, but are hazardous if there is 7 any possibility of leakage to atmosphere or to an aqueous one or more'heat-withdrawal zones, with provision for controlling the temperature in the last-named zones by adding water at a constant rate to the circulating mass and withdrawing a variable' quantity, depending upon the temperature to be maintained. Additional tempera- Patented May 9, 19 6 ture control may be imposed by adding heat to the heat addition zone at a rate controlled by the outlet tempera-;

ture of the'water leaving that zone. Also, the temperature profile across the several heat withdrawal zones may becontrolled by regulating the flow paths of the circulating fluid. w

A specific scribed below is to a liquid-phase hydrocarbon oxidation process in:which heatis to be added to theincomingreactants priorto theirintroductioninto a series of reaction zones, and also is supplied in varying amounts to the last-named zones to maintainthe temperature and temperature profile of the endothermic reaction therein, at the optimum value. Liquid make-up to the circulating system and the inevitable heat losses make up the remainder of the heat load. The reaction conditions are suchthat thetemperature is desirably maintainedwithin the relatively narrow .range of about .6207-640 F,, and the pressure is kept aboveabout 2800 p.s.i.-a. to prevent.

reactant vaporization as well as vaporization of the heating liquid. The nature of the reactants and the materials of: the, tubular heat exchangers in which the reaction oc-{ curs desirably are such thatany failure or leakage should be from the heating liquid side to the reactant side, and

not in the opposite direction. Thus, the pressure 'on the heating liquid side must exceed 2800 p.s.i.a., and has been chosen, in thisexample, to be in the range from about 3000.-3200 p.s'.i.a.- Undenthese conditions, the

properties of ordinary vfresh water, which have been: found suitabletor the system in question, are quite different from those usually encountered at lower pressures;

andtemperatures v and are illustrated graphically in cer-' tain of the drawings herein. Aqueous solutions generallywould havecomparable properties and may be of ad vantage ini cert'ain, applications of the methods of the invention.,.

This invention" will bet note clearly understood it. it

is considered that'essentially three systems are involved:' the process reactor system, which is at aco'ntrolled pressure determined 'thet'reaetion conditions therein; a circulating heating water system, which is not held'at any fixed value, b'ut'is controlled to a value about 200 p. s.i,.; above the reactor system; and a water make-up system or water source; Regarding the make-up water source as a system, it is heldbyrneans of asuitable pumpand pressure controller to a value about circulating Watersy'stem;

Referring now to thepressure conditions for'thejcirc'ulatingwater system, itis controlled by' bleeding or r'emoving water fromv that system at the rate required'to hold the totalvolume of ,water constant. The difiere'ncej between volume. and mass of water should be, kept in mind relative to' their effect on system pressure,s incethe e'ifectof changing temperature level within thefsys tern and changing temperature profile around the system is to destroy any fixed relationship between the mass of' water in,the system and the pressure resulting therefromi For example, when the systemis cold, density of the water" is high (specific gravity about 1.0) anda relativelylarge mass of water is required to fill thesystetn and squeeze -f against the containing piping and vessels to raise pres? sure to the desiredfvalue. As the temperature rises, for example, to a value fnearits critical point, the'densityflde creases rapidly, c ansirig a smaller massof Water to occupy the sar'rie{' volume.f Therefore, watermust be re- I moved from thefsystem'i to maintain the same pressure;. Conversely, if t he temperature in the system .as a whole;

or any'pa'rt of it, is lowered, water m 'b adde'drfo maintain the system pressure. i

Temperature-densityrelat onships oi water whitishperature-levelfare 'suclirth'at largelchangesin rate of addition'of cold water would seriously affect theten't f application of this method that W111 be de-.

50 pebq e the asser s perature profile within the system, and therefore correspondingly affect the pressure. To overcomethis, water is added at a constant rate, establishing a constant heat demandwhich is supplied by the heating zone. Water isbled from the system at a greater or smaller rate as is required by the pressure conditions; Thus, the total mass within the circulating system is controlled by the difference between the constant rate of addition of cold water and the varying rate of withdrawal of hot water.

It is an object of this invention to provide a method ofstabilizing the operation of a heat-transferring system using a liquid medium, for example, water or an aqueous solution, under conditions approaching its critical point. Another object is to provide a method of supplying heat at an accurately controlled rate to an endothermic process in which deviations from normal conditions of operating are self-correcting or at least occur so slowly that a supplementary automatic or manual control may be imposed to attain and maintain equilibrium in both 1 pressure and temperature.

Another object is -to provide a method of operating a closed system for heat interchange in a chemical process with substantially a constant mass of circulating liquid, and desirably water, in the system so' that chang s in temperature of the heat-withdrawal zone are reflected byjsubstantial changes in the system pressure, which changes may be used to restore the system to the desired.

operating conditions.

Another object is to provide a method of controlling the pressure in a closed system carrying a circulating liquid that passes successively through'a heat-addition zone and one or more heat-withdrawal zones by adding liquid to the system at a constant rate and withdrawing liquid at a variable rate, dependingupon' pressure changes in the system.

' These and other objects and advantages will be further apparent from the following description of a'preferred and an alternative embodiment of a system for carrying out the steps of this invention, taken in connection with the attached drawings, which form apart of this specification. i

In the drawings: V

Figure 1 is a curve showing relationship between pressure, temperature, and mass. of water in a circulating system of a given size, in this case, containing 600 cubic feet.

Figure v2 is a similar curve showing a part of the data of'Figure 1, to an enlarged scale.

Figure 3 is a simplified flow diagram of a system embodying this invention and adapted to supply heat to a reaction zone.

, Figure:4 is a simplified flowdiagram of an alternative form of a system embodying thisinvention and adapted to... supply varying quantities of heat to a plurality of heat-withdrawal zones.

,Referring to the drawings, and particularly to Figures 1 and 2, the family of curves there shown illustrates the essential relationships between pressure, temperature, and the mass of a water-filled closedsystem, in thiscase having anarbitrary constant volume of 600 cubic feet, under conditions approaching the critical point for water. The

ordinate scale represents the mass of water in the system and the abscissa scale represents the pressure in the system. It will be noted that, fora temperature increase, for example, from 600 F. to"620 F. at a controlled pressure of 3.400 p.s.i.a., about900 pounds of water must be released from the system. 'Such a condition may be encounteredby a change in the process conditions of the reaction to which heat is being added by the circulating. water system of this example. Conversely, if the heat demand should decrease, or if a change in temperature profile in the several reactors should require'a comparable decrease in average system temperature, 900

pounds of water would berequireduto beadded to maintain the desired pressure and temperature conditions.

Figure 1 shows the relationships just discussed for a temperature range from 100 F. to 700 F. Figure 2 is a portion of the same set of curves, but to an enlarged scale and in the more limited range of 640 to 700 F. Lines A and B illustrate a desired range of pressure conditions for the exemplary system of this specification.

Referring now to Figure 3, which illustrates schematically a very simple embodiment of the invention, reference numeral 10 designates a heat-addition zone, which may be a tubular or other type of furance, with a burner 11 supplied by a fuel line 12 provided with a regulator 13 controlled by. the temperature of the water leaving zone 10 through outlet line 14. Line 14 leads to a heatwithdrawal zone 15 which may be one, or more indirect heat-exchangers or reactors with appropriate lines 16 and 17 to carry reactant fluids into and out of the zone. An example of the type of reaction zone to which this invention is particularly adapted is that of US. Patent 2,722,549, issued November-'1, 19,55, to W. G. Toland, Jr., on an Oxidation Process.

From the heat-Withdrawal zone 15, line 18 carries the circulating water stream to circulating pump 19, from out .the desired release ,ofwater at a variable, rate, as-

willbe apparent to oneskilled; in this art.

in practice, this system has been found to be directionally stable due to the mass of water in the circulating. stream and the heat capacity of the heat-withdrawal equipment and the fluid reactants therein, as well as to the novel method for regulating the mass of water in the closed system.

Reference is now made to Figure 4, which illustrates a more complex embodiment of the invention, including temperature, flow-, and pressure'control means for adjusting the temperature gradient in a plurality of heatwithdrawal zones, certain groupsof which may have somewhat different heat demands. Another characteristic of this arrangement, which will be further apparent from the detailed description below, is the step of storing at a lower pressure the water that is variably withdrawn from the circulating system, and reusing it for making up the constant rate water input. Similar reference numerals have been employed where they are applicable. g

In Figure 4, the heat addition zone to isillustrated as a tubular heater with burners 11 supplied by fuel from any suitable source (not shown) through line 12 and regulated by valve 13 which is responsive to the 7 temperature of water leaving zone 10 through outlet line 14. That line is branched at 3i? so as to provide controlled parallel flowofw'vater through tubular heat exchangers 15a and 15b, and provision is also made by afford accurate controlofternperature gradients in the heat exchangers. 15:1,}1, c, and d.

file Step of storing the water which is withdrawn from the. system at a variablerate dependent upon the ternperature-pressure-mass relationships mentioned above, and-returning it to the system, at a constant rate is carried out, in this example, by means of an appropriately dimensioned surge drum 36, the inlet of which cornmunicates with the circulating water line 18 by means of line 24, part of which desirably forms a heat-exchange coil 37 in drum 36, and a pressure-responsive flow regulator 23. Desirably, that regulator is responsive to the pressure in the water circulating system and is also arranged to be reset as required by the pressure in the reactant feed inlet line 16 so as to maintain a constant excess of pressure in the circulating water system, for example, 100 to 200' psi. above the pressure in the reactant system. As mentioned above, this insures that any leakage will be from the water to the reactant system, and not in the opposite direction.

It Will be apparent, from a consideration of Figures 1 and 2, that surge drum 36 will be required to accommodate a substantial change in liquid level due to water that must be bled off from the circulation system during starting periods and replaced when shutting down, so that a plurality of vertically spaced liquid level controls generally designated 38 are usually required selectively to control water level in that drum. Absolute pressure of about 20 p.s.i.a. in drum 36, in this example, is provided by a vent condensing coil 39 in the top of the drum, supplied by cold water and regulated by pressure controller 40. A valved vent outlet 41 bleeds air and uncondensed gas continuously from the top of the condensing section.

One of the unique features of the circulating pump 19, which operates at pressures of 3000-3400 p.s.i.a. and temperatures up to about 690 F., is the provision for sealing against leakage the shaft 42 connecting the pump and its prime mover generally designated 43. Conventional packings between the shaft and pump housing are not usable under the pressure and temperature conditions just outlined, and the sealing requirement for the shaft 42 is met by providing flow-throttling means, in this case an elongated sleeve 44 having a plurality of successive close-clearance annular constrictions or diaphragms therein, in the manner of a steam turbine shaft labyrinth, with a water inlet connection 45 at the inner end of the labyrinth and a water outlet connection 46 at its outer end. Water from surge chamber 36 passes through outlet line 47 and high pressure make-up pump 48 to a flow-regulator 21 from which it divides at 49, so that part of the flow passes through line 22 to the main water circulating line 20, and the remainder passes through branch line 50 to the sealing water inlet connection 45 of circulating pump 19. Here again the Water flow divides, part passing into the body of pump 19 and the remainder passing through the shaft-sealing labyrinth to the outlet connection 46 and thence through the valved water return line 51 to the low-pressure surge drum 36. Desirably, a difierential pressure regulator 52 is provided to control the division of water between line 22, leading to the main circulating stream, and line 50, leading to the circulating pump sealing inlet 46, by returning the excess from make-up pump 48 to surge chamber 36.

The methods and means described have been quite successful in meeting the requirements of the exemplary system, which uses fresh water under conditions approaching the critical point (3226 p.s.i.a. and 706 F.) by providing for the large changes in density of the water with temperature. The high pressure circulating water system has a substantially fixed volume determined by the several components of the heat-addition zone, heat-withdrawal zone, circulating pump and lines,

6 so that the mass of water therein must vary with both the average water temperature and the temperature profile in the system. This is accomplished by pumping into the system a constant surplus of water and at the same time withdrawing a variable amount dependent on the pressure conditions in the water system and also in the reaction system. Ancillary means for controlling temperature gradient in a plurality of heat-withdrawal zones includes flow controls and pressure-controlled bypasses as described above.

From the foregoing, it will be apparent that an easily balanced and inherently stable flowand temperatureregulating system has been provided, as has been proved in'operation, to carry out the method of this invention,

as defined in the following claims. Although only specific embodiments have been illustrated and described in: detail, it will be apparent that changes can be made without departing from the essential features of the invention. Accordingly, all such modifications as are covered by the appended claims are intended to be included therein.

We claim:

1. A method for transferring heat which comprises circulating a liquid at a temperature substantially above its normal boiling point and under a high superatmospheric pressure through a closed loop system including a heat input zone and a heat withdrawal zone in which heat is transferred from the liquid circulating in the loop, introducing liquid into the loop at a substantially constant rate and withdrawing liquid from the loop at a point in the loop removed from the point of liquid introduction at a variable rate adapted to maintain a pre determined high superatmospheric pressurein the loop.

2. A method for transferring heat which comprises circulating a liquid at a temperature approaching its critical temperature and'under a high superatmospheric pressure through a closed loop system including a heat input zone and a heat withdrawal zone in which heat is transferred from the liquid circulating in the loop, introducing liquid into the loop at a substantially constant rate and withdrawing liquid from the loop at a point in the loop removed from the point of liquid introduction at a variable rate adapted to maintain a predetermined high superatmospheric pressure in the loop.

3. A method for transferring heat which comprises circulating water at a temperature above about 600 F. and at pressures above about 2800 p.s.i.a. through a closed loop system including a heat input zone and a heat Withdrawal zone in which heat is transferred from the Water circulating in the loop, introducing water into the loop at a substantially constant rate and withdrawing water from the loop at a point in the loop removed from the point of water introduction at a variable rate adapted to maintain a predetermined pressure above about 2800 p.s.i.a. in the loop. 1

References Cited in the file of this patent UNITED STATES PATENTS 1,682,674 Hedlund Aug. 28, 1928 1,925,222 Abendroth Sept. 5, 1933 2,061,605 Yoder Nov. 24, 1936 2,262,194 Newton Nov. 11, 1941 2,491,576 Oaks Dec. 20, 1949 2,756,739 Schaub July 31, 1956 FOREIGN PATENTS 914,677 France Sept. 20, 1945 

